Machines with automatic transmission employ a torque converter. The torque converter is a hydrodynamic fluid coupling that typically transfers a rotational torque from a prime mover to a driven load, via a transmission. The torque converter typically includes an impeller, a turbine, and a stator. The impeller is connected to a driving shaft of the prime mover, such as a crank shaft of an internal combustion engine. The turbine is connected to an output shaft, such as a transmission input shaft. In the basic fluid coupling design of the torque converter, the fluid flow returning from the turbine to the impeller opposes a rotation of the impeller, leading to a loss of efficiency and generation of heat.
In modern torque converter designs, the stator is provided to redirect the returning fluid flow such that it aids the rotation of the impeller, instead of opposing it. This results in energy that is recovered from the returning fluid and added to the energy supplied by the prime mover to the impeller. This also increases the fluid flow which is directed to the turbine, producing an increase in output torque. Since the returning fluid is traveling in a direction opposite of impeller rotation the stator may try to counter-rotate as it forces the returning fluid to change direction; to prevent this effect a one way stator clutch is provided to prevent the stator from rotating. As the turbine speed increases and approaches the impeller speed in response to reduced load on the output shaft, the fluid flow from the turbine to the impeller decreases. At some point the flow being returned from the turbine will decrease enough, in the reverse direction, and attempt to rotate the stator in the opposite direction so as to unlock the stator clutch. When the stator clutch unlocks the impeller, turbine and stator will all rotate at roughly the same speed. Furthermore, a lock-up clutch is provided to physically couple the impeller and turbine, and effectively change the torque converter into a purely mechanical coupling. This result in no slippage, and virtually no power loss.